Signal isolators for input measuring currents of, e.g., 0 or 4 mA up to 20 mA or for input measuring voltages of, e.g., 0 to 1 volt are known.
A signal isolator whose power loss is to be reduced is known from DE 10 2007 006 503 A1. This signal isolator has an input for a measuring current. The input is connected to a downstream DC transformer with isolated potential which transmits the measuring current to an output circuit. The output circuit comprises a linear regulator which includes an operational amplifier and an output transistor and functions as a voltage-to-current converter. The output circuit of the linear regulator includes the output terminals of the signal isolator to which a load can be connected. The output circuit further comprises a switching regulator that is arranged between the output terminals. Switching regulator and linear regulator are implemented as separate functional units. The switching regulator receives, from the linear regulator, a control signal which regulates the switching regulator such that the output voltage of the linear regulator is minimized.
Also, a signal isolator is known from DE 10 2010 061 433 A1, which comprises, as an input stage, an inverter, a transformer, and a rectifier, via which an input measuring current is supplied to a linear regulator. The linear regulator comprises an operational amplifier and an output transistor to which a load can be connected. To reduce the power loss of the output transistor, the conventional signal isolator has a control device implemented as a switching power supply which controls an auxiliary voltage in a manner so that the power loss of the output transistor of the linear regulator is substantially independent of a load connected in the operating state. Linear regulator and switching power supply are separate functional units.
The invention is based on the object to provide a signal isolator with circuitry of significantly reduced complexity compared to the known signal isolators, so that the power loss of a signal output stage of the signal isolator can be further reduced.